Chitin is the second most abundant polysaccharide in nature, after cellulose. It is comprised of N-acetylated glucosamine residues linked via .beta.-1,4 glycosidic bonds, and in nature, has an estimated molecular weight of 1 to 2.times.10.sup.6 daltons. FIG. 1 shows a representative structure of chitin. When the degree of N-acetylation is low (&lt;40%), this polysaccharide is referred to as chitosan. Chitin occurs in nature as a structural component of the insect and crustacean exoskeleton, as well as the fungal cell wall. It is almost always associated with proteins in its role as a fibrous strengthening material.
Chitin was once believed to be an intractable material, but has been found relatively recently to be susceptible to various chemical modifications, especially at the 3 and 6-O positions. These derivatives include among others, various alkyl chitins, carboxymethyl chitin, hydroxyalkyl chitins, cyanoethylchitins and chitin xanthogenates. Chemical reactions used to produce these derivatized chitin are usually carried out heterogenously with alkali-chitin or homogenously, using a solution of chitin in dimethylacetamide (DMAc) in the presence of 5% LiCl. Controlled enzymic or acidic degradation of chitin yields chito-oligosaccharides which are also suitable starting materials for similar derivatization reactions.
The primary source of chitin is the shells of crab and shrimp, an abundant by-product of the seafood processing industry. The crude purification procedure includes with dilute hydrochloric acid and removal of protein impurities by alkali treatment. The relatively harmless reagents and waste products involved in chitin purification provides an ecological advantage over the production of cellulose. Potential applications of chitin include use as artificial skin, diet supplement, surgical sutures and in drug delivery systems. On the other hand, the moistening and gel forming properties of chitosan and its derivatives make it usefuil for food and cosmetics applications.
Chitin gels have been referred to in the literature (Hirano et al, in Biotechnology and Polymers, Ed. C. G. Gebelein, Plenum Press, New York, 1991, pp. 181-188). A partially O-acetylated chitin gel was prepared by dissolving chitin with stirring, in a N,N dimethylacetamide/5% LiCl solvent system, adding acetic anhydride and pyridine, and permitting the solution to stand at 100.degree. C. for 6 h. Deacetylation of the latter gel with aqueous NaOH yielded a chitin gel. Hirano also reported the acetylation of a chitosan gel to a chitin gel by reacting the gel with acetic anhydride and described the production of chitin films by air-drying or freeze-drying of chitin gels.
Chitin films have also been referred to in literature. Methods of preparing chitin films vary in terms of casting solution, coagulating agent and/or drying method employed. A chitin membrane was prepared by coagulation of a dimethylacetamide/N-methylpyrollidone/LiCl solution of chitin using 2-propanol, followed by immersion of the membrane in water (S. Aiba et al in Preparation and Properties of Dialysis Membranes, Chitin in Nature and Technology, Ed. R. Muzzarelli, C. Jeuniaux, G. W. Gooday, Plenum Press, New York, 1986, p. 396 prepared). Chitin films have also been prepared by pressing a powdered sample at room temperature followed by heat-treatment in vacuum at 50.degree. C. for three days (M.Kakizaki et al in Molecular Motion and Dielectric Relaxation in Chitin and Acylchitins, Chitin in Nature and Technology, Ed. R. Muzzarelli, C. Jeuniaux, G. W. Gooday, Plenum Press, New York, 1986, p. 39). A chitin film has also been prepared by casting a solution of chitin in N,N dimethylacetamide/5% LiCl and allowing evaporation of the solvent, leading to coalescence of the film (Rutherford and Dunson in The Permeability of Chitin Films to Water and Solutes, Chitin, Chitosan and Related Enzymes, Ed. J. P. Zikakis, Academic Press, New York, 1984, p.136). The coalesced film was washed in acetone and water, blot dry, placed between paper towels and pressed in a book.
6-O-carboxymethyl-chitin is a derivatized form of chitin that is extremely water soluble when the degree of substitution on the 6O group is greater than 0.6. The structure of this derivative is shown in FIG. 2. The derivative is obtained by the reaction of monochloroacetic acid on alkali chitin. This 6-O-carboxymethyl-chitin is very water soluble and is not a gel. Lower degrees of substitution renders the 6-O-carboxymethylchitin insoluble in water. Gels can be obtained from these water insoluble 6-O-carboxymethyl-chitins by first dissolving in 90% formic acid and allowing the evaporation of the acid. This method is tedious, utilizes large amounts of a dangerous acid and the gels obtained do not reversibly swell in water (A.C.A. Wan, E. Khor, J. M. Wong, G. W. Hastings, Promotion of Calcification of Carboxymethyl-chitin Discs", Biomaterials, Britain, in press, March, 1996).
However, There has been no mention in the literature (Hirano et al, in Biotechnology and Polymers, Ed. C. G. Gebelein, Plenum Press, New York, 1991, pp. 181-188) of reacting chitin in the dry form to obtain a gel.
Furthermore, there has also been no mention of obtaining a chitin film by solvent drying. In the method reported by Rutherford and Dunson, (Rutherford and Dunson in The Permeability of Chitin Films to Water and Solutes, Chitin, Chitosan and Related Enzymes, Ed. J. P. Zikakis, Academic Press, New York, 1984, p.136), acetone was used solely for the purpose of washing and not drying the film. The washing procedure is followed by a final wash with water prior to drying to obtain the film. Acetone was not used as a drying agent in a solvent-drying process, which is another object of this present invention.
Finally, there is no known example of chemically reacting chitin in the dry form to produce materials with properties such as reversible swellability in water.